The present invention relates to an optical head apparatus for writing data onto an optical disk, or for reading data recorded on the optical disk, by employing an optical beam irradiated from a light-emitting element. More specifically, the present invention relates to a detecting device for use in focusing an objective by a servomechanism in the optical head apparatus, and to a lens actuator for the optical head which supports an objective so that it can move in a focusing direction and a tracking direction perpendicular to the focusing direction, and drives it in those two directions.
In general, optical heads using semiconductor lasers have been used to write data onto optical disks or read the data therefrom. In such an optical head, which converges a laser beam by an objective and irradiates the converged beam onto a predetermined position on the optical disk, irradiation of the laser beam must be controlled in such a manner that the irradiated light spot focuses on the signal surface of the optical disk to ensure that the data is read or written correctly, so the objective of the optical head is focused by a servomechanism.
Conventionally, various different methods have been proposed as detection devices for use in focusing the objective by a servomechanism. One example is a device which utilizes astigmatism. This device has a condenser lens, a cylindrical lens for converging rays of light in one direction alone, and a four-part photodetector, all these components being disposed in the optical path of light reflected from the optical disk in order to form an astigmatic image. The four-part photodetector is mounted at a location at which a beam whose cross-sectional shape is a true circle appears when the laser beam is accurately focused on the signal surface by the objective. Therefore, when the objective is moved closer to or further away from the optical disk, the beam shape is changed to an ellipse which is elongated in either of two directions perpendicular to each other by the action of the cylindrical lens, and a focusing error signal can be detected by first adding the outputs of diagonals of the four-part photodetector, then obtaining the difference therebetween.
This known technique, therefore, requires a cylindrical lens in addition to a condenser lens, thus increasing the number of parts and hence the optical path. Further, the optical parts in the detection section must be positioned with high accuracy.
In order to converge and focus the laser beam by the objective accurately onto a predetermined position on the optical disk, the objective is movably supported so that it is movable in the focusing direction and the tracking direction perpendicular to the focusing direction, and can be driven in those two directions. Currently, electromagnetic force generated by the interaction of the current flowing through a coil and the external magnetic fields is utilized to drive the objective. Various supporting structures for the objective have been proposed, the typical examples including the axial rotation and sliding method and the spring method. A supporting device which adopts the shaft sliding method has a lens casing with a lens mounted on a portion of the casing which is separated from the central shaft. The lens casing is capable of sliding along the central shaft and rotating about the central shaft within a predetermined range. In the spring method, a freely movable lens casing is supported by a plurality of springs.
In both shaft sliding and leaf spring supporting methods, however, it is difficult to restrict the generation of jitter and to reduce frictional resistance at the same time. It is desirable for the lens casing to be supported in such a manner as to movable solely in the focusing direction (in the same direction as the optical axis of a lens) and the tracking direction perpendicular to the focusing direction and not to be movable in other directions, because movement of the lens casing in the direction perpendicular to the focusing and tracking directions generates noise associated with the time axis, i.e., jitter. In addition, movement of the lens casing in the focusing and tracking directions must be conducted with a minimal degree of friction.
In the shaft sliding method, the shaft is area-contacted with the lens casing. Therefore, to restrict the generation of jitter, the backlash of the fitting of two components must be minimized, this resulting in an increase in the frictional resistance. In the spring method, since the lens casing is supported by a plurality of springs alone, frictional resistance is low. However, movement in a direction other than the focusing and tracking directions is not restricted, furthermore wave aberration at the recording surface is generated, and jitter thereby readily increases.